Responding to written questions from a United States senator about the effects of concussions in hockey, N.H.L. Commissioner Gary Bettman continued to deny a link between concussions and the brain disease chronic traumatic encephalopathy, or C.T.E. He blamed the news media for fanning fear of the long-term effects of head injuries and defended the N.H.L.'s "more measured approach" to the growing science of concussions.

When the former N.H.L. enforcer Todd Ewen died in September, reportedly of a self-inflicted gunshot, his brain was sent to researchers. Years of memory loss and undiagnosed depression led to speculation that Ewen, 49, had chronic traumatic encephalopathy, a degenerative brain disease caused by repeated blows to the head.

DEREK BOOGAARD was scared. He did not know whom he would fight, just that he must. Opportunity and obligation had collided, the way they can in hockey. His father bought a program the night before. Boogaard scanned the roster, checking heights and weights. He later recalled that he barely slept.